A wide variety of spinal fixation systems are currently in use by orthopedic surgeons. These various systems are widely used to help safely secure and stabilize the spine to correct deformities. In addition, spinal implant systems can aid in a healing process from trauma, or assist in the treatment of degenerative conditions. These types of implants are designed to resist post operative fatigue and failure until bone fusion occurs.
In general, two or more bone screws and/or hooks are secured to the vertebra to be stabilized. After installation, the screws or hooks extend outward roughly perpendicular to the spinal column. They are then coupled to a spinal fixation rod which is placed roughly parallel to the spinal column, thus restricting the movement of the stabilized vertebra. In most applications, two spinal fixation rods are provided, each coupled to bone screws or hooks provided on both sides of the spinal column. The two spinal rods are often attached to one another with transverse connecting pieces.
It can be appreciated that the coupling means securing the bone screws or hooks to the spinal fixation rod is important to both the effectiveness and ease of installation of such spinal fixation systems. Various coupling means have been used. In some systems, the bone screw or hook incorporates a "Y" shaped head containing a lateral fixation rod engagement groove. An eyebolt is provided which fits between the posts forming the "Y", and a nut is tightened onto the eyebolt which holds the "Y" shaped head against the fixation rod in the lateral engagement groove. This type of coupling means is exemplified in U.S. Pat. No. 5,246,442 to Ashman et al.
Eyebolt type coupling systems have several disadvantages. The eyebolt may score the fixation rod, leading to premature failure. Also, the surgeon is required to bend the rod to conform to the lordotic (concave) or kyphotic (convex) curves in the surgical area to position the rod engaging portion of the screw or hook exactly perpendicular to the rod. Furthermore, because such eyebolts attach to the spinal rod in a "side-tightening" manner, access to the nut during surgery is limited, making the process of tightening the nut laborious and time consuming.
In response to these problems, various forms of adjustable and "top tightening" coupler systems have been devised. For example, U.S. Pat. No. 5,047,029 to Aebi, et al. describes a coupling device which incorporates a threaded compressible collet which clamps down on the shaft of a bone screw as the collet is drawn into an eye by turning a nut threaded onto the collet. U.S. Pat. Nos. 5,129,899 to Small et al. and 5,092,893 to Smith both describe bone screws incorporating upper threaded portions that extend through a slot in a plate, wherein the plate acts as a fixation device. The screw is secured directly to the plate by threading a top tightening nut onto the upper portion of the bone screw which protrudes through the slot in the plate. Although they are improvements over the side tightening eyebolt type coupling, these systems remain imperfect. For example, angular adjustability is not appreciably present in either the Small or Smith systems, and the Aebi device requires independent attachment to the rod and the bone screw.
Accordingly, it would be beneficial to provide a coupling device for a bone fixation system incorporating the benefits of wide angular adjustability as well as allowing single nut top access tightening.